The volume of communication information data tends to greatly increase with the advancement of the information society necessitating the widespread use of wavelength division multiplexing (WDM) transmission in the field of telecommunications. WDM transmission enables to transmit optical signals of plurality of wavelengths through a single optical fiber.
The erbium doped fiber amplifier (EDFA) is today developed and applied to amplify the optical signals at points of relay in a WDM transmission system. The EDFA does not require a process of transforming optical signals into electric signals at each wavelength, accelerating the spreading of WDM transmission worldwide.
In the meantime, Raman amplifiers with the Raman amplification are of great interest as a new optical amplifier. The Raman amplification is a system which amplifies light using the stimulated emission in Raman scattering. The amplification efficiency depends largely on the non-linearity of an optical fiber. As usual, the more the non-linearity is enhanced, the more the efficiency is improved.
FIG. 23 is a diagram for a example of optical transmission system applying the Raman amplification. The output of the Signal Light Sources 4al to 4an have different wavelengths of signal lights and are multiplexed by Optical Multiplexer 15.
The output of the Pump Light Sources 3al to 3an have different wavelengths of pump lights, and are in a multimode lasing. The pump lights from Pump Light Sources 3al to 3an are mixed by Optical Multiplexer 16. The pump lights and signal lights (WDM signal lights) are multiplexed by Optical Multiplexer 10 and led into Optical Fiber 8 in the optical transmission line.
The WDM signal lights in Optical Fiber 8 propagate as being Raman amplified till the receiving end, and then are demultiplexed for the respective wavelengths by Optical Demultiplexer 9, and received by Optical Receivers 7al to 7an. 
FIG. 23 is an example of co-propagation pumping system in which the pump lights for the Raman amplification propagate in the same direction as the transmission signal light in an optical fiber (Optical Fiber 8). By contrast, a second example, as shown in FIG. 24, is a counter propagation pumping system in which pump lights propagate in the reverse direction to the transmission signal lights in an optical fiber (Optical Fiber 8).
Light Sources 13al to 13an in FIG. 24 output pump lights, which are multiplexed by Optical Multiplexer 26. The pump lights were coupled to the Optical Multiplexer 20 to propagate in the reverse direction to signal lights.
Moreover, a third example shown in FIG. 25 is a bi-directional pumping system in which pump lights propagate in both directions in an optical fiber (Optical Fiber 8).
Concurrently in the same diagram, the pump lights from Pump Light Sources 3al to 3an in FIG. 25 are multiplexed by Optical Multiplexer 16, and those from Pump Light Sources 13al to 13an are multiplexed by Optical Multiplexer 26. The pump lights propagating in the same and reverse direction to the transmission signals are multiplexed with transmission signals by Optical Multiplexer 10 and 20, respectively and then fed into Optical Fiber 8. It is preferable to apply a bi-directional pumping for making the light intensity longitudinally more uniform throughout the Optical Fiber 8.
If Optical Fiber 8 is made of silica, the peak gain for the Raman amplification appears at a frequency 13 THz lower than the frequency of the pump light source (wavelength about 100-110 nm longer). In brief, in the Raman amplification there are amplified signal lights at a 100-110 nm long wavelength from the pump light wavelengths.
Consequently, as for an optical transmission system in a wavelength band of 1.5 μm, for instance, a maximum Raman gain can be obtained for signal lights at 1580 nm when input pump light is at 1480 nm.
The optical fiber transmission characteristics depend on the transmission loss and chromatic dispersion. A single mode fiber offers a single mode of signal propagation as free from mode dispersion, and accordingly its transmission band is limited by material dispersion and waveguide dispersion. Conventional single mode fibers have a zero dispersion at a wavelength of 1.3 μm band (hereinafter called at 1.3 μm) or thereabout, and a lowest transmission loss in a wavelength band of 1.5 μm (hereinafter called in 1.5 μm). Against this backdrop, a plan for Dispersion Shifted Fiber (DSF) was proposed in an attempt to shift a zero dispersion from 1.3 μm band to 1.5 μm band.
It is difficult to shift the zero dispersion from 1.3 μm to 1.5 μm by changing the material dispersion of an optical fiber, and is obtained by changing the wave guide dispersion. In short, an refractive index profile adjustment to the core and cladding resulted in shifting the zero dispersion to 1.5 μm.
The optical fiber transmission characteristics can be more improved generally by further reducing chromatic dispersion. Yet, launching WDM signals into an optical fiber with too low chromatic dispersion induces unwanted waves in consequence of Four Wave Mixing (FWM), leading into a fault of the increase in the inter-channel cross talk. This fault was solved by the proposal of Non-Zero Dispersion Shifted Fiber (NZDSF).
One of the research and development tasks for the WDM transmission is to pursue a broader transmission band of wavelength. The main range of operating wavelengths has been made up of the C-band (1530-1565 nm) and the L-band (1565-1625 nm). Recently, a reduced dispersion slope NZ-DSF to cover up-to the S-band (1460-1530 nm) is substantiated and proposed in papers at “ECOC '01 PD A1-5 (2001), OECC '02 11D1-2 (2002)”.
However, even a reduced dispersion slope NZ-DSF is subject to limitations on its shorter-wavelength range by the zero dispersion wavelength, and on the longer wavelength range by the cumulative dispersion Accordingly, the wavelength band for transmission was limited to around 200 nm. In addition, the zero-dispersion wavelength of the NZ-DSF limits pump light wavelength band for the Raman amplification, almost unworkable in the S-band.
One purpose of the present invention is to provide optical fibers suitable for the WDM transmission in a broad wavelength range between the E-band and U-band, also to allow for Distributed Raman Amplification in the S-band and to provide optical fibers with lowered bending loss and suppressed non-linearity